1. Field
The present invention relates to a reflective mask and a method for manufacturing the reflective mask; and, in particular, relates to a reflective mask utilized in a semiconductor fabrication device using EUV lithography whose light source is extreme ultraviolet ray (hereinafter, represented as “EUV”), and a method for manufacturing the reflective mask.
2. Description of the Related Art
Description of EUV Lithography
Associated with miniaturization of semiconductor devices, the demand of miniaturization has increased for photolithography technique. As a part of realizing miniaturization with photolithography technique, replacement is already being made in the exposure method of lithography, from exposure using conventional ArF excimer laser light having a wavelength of 193 nm to exposure using light in the EUV range with a wavelength of 13.5 nm. EUV lithography has to be performed in vacuum since the wavelength of a light source is short and the light absorption property is very high with EUV lithography. In addition, the refractive indices of most substances are values slightly smaller than 1 in the wavelength range of EUV. Therefore, the conventionally used transmission type dioptric system cannot be used in EUV lithography, and a catoptric system is used. Thus, it is necessary to use a reflective mask as a photomask (hereinafter, referred to as mask) that becomes the master copy, since the conventional transmissive mask cannot be used.
Description of Reflective Mask and Blank Structure
Such a reflective mask is created based on a substrate referred to as a reflective mask blank. The reflective mask blank is obtained by, sequentially forming, on a low thermal-expansion substrate, a multilayered reflective layer having high reflectance against the wavelength of a exposure light source, and an absorption layer configured to absorb the wavelength of the exposure light source, and further forming, on the reverse surface of the substrate, a reverse-surface conductive film for electrostatic chuck fixing in an exposure machine. There are also reflective masks with a structure having a buffer layer between the multilayered reflective layer and the absorption layer. When processing the reflective mask blank into a reflective mask, the absorption layer is partially stripped together with a buffer layer if the structure has a buffer layer, using EB lithography and etching technology to form a circuit pattern consisting of absorption parts and reflective parts. Optical images reflected by the reflective mask produced in such manner are transcribed onto a semiconductor substrate via a catoptric system.
Description of Reflectance and Film Thickness of Absorption Layer of Reflective Mask
With an exposure method using the catoptric system, deflection using a transmissive beam splitter is not possible. Therefore, the reflective mask has a disadvantage of being unable to design incident light to the mask and reflected light therefrom to be on the same axis, and since incident light is emitted at an incidence angle (ordinarily 6°) inclined by a predetermined angle from a perpendicular direction with respect to the mask surface, a shadow of a pattern itself is generated when the film thickness of the absorption layer is large. Since the reflection intensity at the shadowed part is smaller than a part that is not shadowed, a reduced contrast is obtained, resulting in a transcription pattern having blurred edge parts and deviation from a designed size. This is referred to as shadowing, and is a fundamental problem for reflective masks.
In order to prevent blurred edge parts of the pattern and deviation from the designed size, reducing the film thickness of the absorption layer and lowering the height of the pattern are effective. However, reducing the film thickness of the absorption layer film results in deterioration in the light-shielding ability at the absorption layer, reduction in transcription contrast, and deterioration in accuracy of the transcription pattern. If the absorption layer is too thin, the contrast necessary to maintain accuracy of the transcription pattern cannot be obtained.
As described above, since it becomes a problem if the film thickness of the absorption layer is too large or too small, the film thickness at present is generally set between 50 to 90 nm, and the reflectance of EUV light (extreme ultraviolet light) at the absorption layer is about 0.5 to 2%.
Description of Multiple Exposures of Adjacent Chips
On the other hand, when forming a transcription circuit pattern on a semiconductor substrate such as a silicon wafer using a reflective mask, chips with multiple circuit patterns are formed on a single semiconductor substrate. In some cases, there is an overlapping area of chip outer circumferential portions of adjacent chips. This is due to chips being arranged in high density for the purpose of improving productivity based on an intention of increasing the number of chips that can be obtained from a single wafer. In this case, the overlapping area of the chip outer circumferential portions is exposed for multiple times (at maximum, four times) (multiple exposures). The chip outer circumferential portions of the transcription pattern correspond to outer circumferential portions of the reflective mask, and the outer circumferential portions of the reflective mask are portions where an absorption layer is formed, ordinarily. However, as described above, since the reflectance of EUV light at the absorption layer is about 0.5 to 2%, there has been a problem of the chip outer circumferential portions being sensitized due to multiple exposures. Therefore, it has become necessary to provide, at the chip outer circumferential portion on the mask, an area (hereinafter, referred to as a light-shielding frame) that has a higher shielding ability against EUV light than an ordinary absorption layer and that has a reflectance of 0.3% or lower.
In order to solve such a problem, reflective masks having a light-shielding frame with high light-shielding ability against exposure light source wavelengths are proposed, including a reflective mask having formed therein a trench that is dug from an absorption layer into a multilayered reflective layer of the reflective mask, a reflective mask having formed therein a film that has a larger film thickness than an absorption film at a circuit pattern area, and a reflective mask in which reflectance of a multilayered reflective layer is reduced by laser irradiation or ion implantation on the reflective mask (e.g., Patent Literature 1).
Citation List
    [PTL 1] Japanese Laid-Open Patent Publication No. 2009-212220